Synchronized C−H activations at proximate dinuclear Pd2+ sites on silicotungstate for oxidative C−C coupling

Carbon−carbon (C−C) coupling is critically important in organic synthesis. Direct C−C coupling to replace two C−H bonds is preferred over coupling of two prefunctionalized C−intermediates but is synthetically challenging. While such coupling is feasible through homogeneous catalysis, the mechanism regarding how the two C−H bonds are activated and coupled by heterogeneous active metal atoms remains not well understood. This work demonstrates the need for a proximate metal−metal dimer site to facilitate heterogeneously catalyzed C−C coupling reactions. We demonstrate that dinuclear Pd2+ sites in (Pd2+)2-silicotungstate (Pd2ST) catalyzed oxidative C−C coupling of 2-methylfuran by O2 through synchronized double C−H activations under ambient conditions, selectively producing 5,5′-dimethyl-2,2′-bifuran (DMBF). Mononuclear Pd2+ ions in Pd H ST and H ST are not active. The 13C NMR and DRIFT spectroscopies of adsorbed 13CO, combined with DFT and theoretical 13C NMR calculations, determined that dinuclear Pd2+ ions are separated by ∼3.5 Å on Pd2ST and 3.1 Å in the Pd2+-C(=O)-Pd2+ complex. XRD and TEM are used to confirm that the most active Pd2ST/SiO2 catalyst has near monolayer dispersion. 29Si MAS NMR is used to confirm the presence of the silicotungstate structure after calcination. The original silicotungstate Keggin structure is maintained after the Pd2ST/SiO2 is calcined.